Question: In history class, the girl to boy ratio is $9$ to $7$. If there are a total of $64$ students, how many girls are there?
A ratio of $9$ girls to $7$ boys means that a set of $16$ students will have $9$ girls and $7$ boys. A class of $64$ students has $4$ sets of $16$ students. Because we know that there are $9$ girls in each set of $16$ students, the class must have $4$ groups of $9$ girls each. There is a total of $36$ girls in history class.